Atrial fibrillation (AF) is a complex arrhythmia with a variety of underlying molecular and structural mechanisms contributing to a vulnerable AF substrate. The complexity of AF substrate seems to be reflected in the characteristics of AF electrograms (EGMs), with AF EGM morphology in paroxysmal AF being different than in more persistent AF. However, the precise structural and functional mechanisms that lead to the formation of AF EGMs have not been well elucidated. The need for a better understanding of the mechanisms underlying AF EGM formation is heightened by several recent descriptions of regions of high-frequency activity during AF called complex fractionated atrial EGMs (CFAEs) (Nademanee K, McKenzie J, Kosar E, et al. A new approach for catheter ablation of atrial fibrillation: mapping of the electrophysiologic substrate.” J. Am. Coll. Cardiol. 43:2044-53 (2004)). Several recent reports suggest that ablation of CFAEs seems to increase AF ablation success (Hayward R M, Upadhyay G A, Mela T et al. “Pulmonary vein isolation with complex fractionated atrial electrogram ablation for paroxysmal and nonparoxysmal atrial fibrillation: A meta-analysis,” Heart Rhythm. 8:994-1000 (2011)).
In the setting of structural heart disease, specifically heart failure (HF), a variety of mechanisms (for example, changes in ion-channel expression and gap junction distribution, inflammation, oxidative stress, and a variety of structural changes) are thought to contribute to the creation of a vulnerable AF substrate.
Oxidative stress is attributed to oxygen derivatives with instabilities and increased reactivity, O2−, H2O2, and OH−, that are generically categorized as “reactive oxygen species” (ROS) (Maejima Y, Kuroda J, Matsushima S, Ago T, Sadoshima J. “Regulation of myocardial growth and death by NADPH oxidase,” J. Mol. Cell. Cardiol. 50:408-16 (2011). While ROS at low doses mediates physiological functions such as growth, differentiation, and metabolism (id.), excess ROS damages DNA, protein and lipids, and causes cell death in cardiomyocytes (id.). A wealth of research data points to increased oxidative stress as a key driver of the cardiac remodeling caused by chronic pressure overload, loss of functional myocardial tissue, or AF (Kohlhaas M, Maack C. “Interplay of defective excitation-contraction coupling, energy starvation, and oxidative stress in heart failure.” Trends Cardiovasc. Med. 21:69-73 (2011); Maulik S K, Kumar S. “Oxidative stress and cardiac hypertrophy: a review,” Toxicol. Mech. Methods 22:359-66 (2012)). Chronic ROS elevation also activates a variety of signaling pathways such as the TGF-β1 and MAP kinase subfamilies (Hori M, Nishida K. “Oxidative stress and left ventricular remodelling after myocardial infarction,” Cardiovascular Research 81:457-64 (2009)), including ERK, JNK (Tsai K H. Wang W J. Lin C W, et al. “NADPH oxidase-derived superoxide anion-induced apoptosis is mediated via the JNK-dependent activation of NF-kappaB in cardiomyocytes exposed to high glucose,” J. Cell. Physiol. 227:1347-57 (2012)) and p38-kinase; these pathways are important in the creation of structural changes in the heart (for example, fibrosis).
Recent evidence indicates that oxidative stress also contributes to structural and electrical remodeling in AF. Significant oxidative damage occurs in appendages of AF patients undergoing the Maze procedure (Mihm M J, Yu F, Carnes C A et al. “Impaired myofibrillar energetics and oxidative injury during human atrial fibrillation,” Circulation 104:174-80 (2001). Dogs with sustained AF were shown to have an increase in protein nitration, suggesting enhanced oxidative stress (Carnes C A, Janssen P M, Ruehr M L, et al. “Atrial glutathione content, calcium current, and contractility,” J. Biol. Chem. 282:28063-73 (2007); Carnes C A, Chung M K, Nakayama T, et al. “Ascorbate attenuates atrial pacing-induced peroxynitrite formation and electrical remodeling and decreases the incidence of postoperative atrial fibrillation” Circ. Res. 89:E32-8 (2001)). In AF induced by rapid atrial pacing, there was an increase in O2− production and NADPH oxidase and xanthine oxidase activity in the LA (Dudley S C, Jr., Hoch N E, McCann L A et al. “Atrial fibrillation increases production of superoxide by the left atrium and left atrial appendage: role of the NADPH and xanthine oxidases,” Circulation 112:1266-73 (2005)). NADPH oxidase (NOX2) is a major source of atrial ROS in patients with AF (Kim Y M, Guzik T J, Zhang Y H et al. “A myocardial Nox2 containing NAD(P)H oxidase contributes to oxidative stress in human atrial fibrillation. Circ Res. 97:629-36 (2005)). More recently, atrial sources of ROS have been shown to vary with the duration and substrate of AF, with NADPH oxidase being elevated early in AF and with mitochondrial oxidases and uncoupled NOS being noted in long standing AF (Reilly S N, Jayaram R, Nahar K et al. “Atrial sources of reactive oxygen species vary with the duration and substrate of atrial fibrillation: implications for the antiarrhythmic effect of statins” Circulation 124:1107-17 (2011)).
ROS are generated within cells by the mitochondrial electron transport chain, the xanthine oxidase/dehydrogenase system, ‘uncoupled’ nitric oxide synthases (NOSs), cytochrome P450 and NADPH oxidases. The NADPH oxidase enzyme family are a major source of cardiovascular ROS (Murdoch C E, Zhang M, Cave A C, Shah A M. “NADPH oxidase-dependent redox signalling in cardiac hypertrophy, remodelling and failure,” Cardiovasc. Res. 71:208-15 (2006); Cave A C, Brewer A C, Narayanapanicker A, et al. “NADPH oxidases in cardiovascular health and disease,” Antioxid. Redox. Signal. 8:691-728 (2006)), with NOX2 being the dominant NADPH isoform in HF (Maejima Y, Kuroda J, Matsushima S, Ago T, Sadoshima J. “Regulation of myocardial growth and death by NADPH oxidase,” J. Mol. Cell. Cardiol. 2011; Murdoch C E, Zhang M, Cave A C, Shah A M. “NADPH oxidase-dependent redox signalling in cardiac hypertrophy, remodelling and failure,” Cardiovasc. Res. 71:208-15 (2006); Cave A C, Brewer A C, Narayanapanicker A et al. “NADPH oxidases in cardiovascular health and disease,” Antioxid. Redox. Signal. 8:691-728 (2006)). However, more recent studies indicate that NOX4 in mitochondria plays an essential role in mediating oxidative stress during pressure overload-induced cardiac hypertrophy (Kuroda J, Ago T, Matsushima S, Zhai P, Schneider M D, Sadoshima J. “NADPH oxidase 4 (Nox4) is a major source of oxidative stress in the failing heart,” Proc. Natl. Acad. Sci., USA 107:15565-70 (2010); Nabeebaccus A, Zhang M, Shah A M. “NADPH oxidases and cardiac remodelling,” Heart Fail. Rev. 16:5-12 (2011); Zhang M, Brewer A C, Schroder K et al. “NADPH oxidase-4 mediates protection against chronic load-induced stress in mouse hearts by enhancing angiogenesis,” Proc. Natl. Acad. Sci., USA 107:18121-6 (2010)).
Of the mechanistic changes that occur in the HF atrium, the generation of reactive oxygen species (ROS) is considered to be especially important in creating conditions conducive to the genesis and maintenance of AF. Yet fundamental information remains lacking about how best to detect ROS-enriched tissues or ROS-damaged tissues (“ROS-associated tissues”) in AF EGM and how that information can be used to perform directed ablation of ROS-associated tissue to improve AF ablation success.